The present invention relates to mobile radio-frequency communication systems, and in particular to improvements in positioning and tracking systems for mobile radio-frequency transceivers such as cellular telephones. In a further aspect the invention concerns a method of finding the position of a mobile radio-frequency transceiver in a communications system.
Two important concepts in understanding the modes of operation of a positioning system are xe2x80x9cremote-positioningxe2x80x9d and xe2x80x9cself-positioningxe2x80x9d. In remote-positioning, a central station works out the location of the mobile. In self-positioning, the mobile works out its own location using data supplied by the station.
Two important modes of operation are radial remote-positioning and hyperbolic self-positioning.
Radial remote-positioning uses measurements of round trip time between a number of base stations and a mobile telephone. The distance between each base station and the mobile telephone can then be calculated by using the fact that radio waves propagate at the speed of light. There are a number of ways of measuring the round trip time, one way being the standard timing advance measurements made by base stations operating with the Global System Mobile (GSM) mobile telephony standard.
In operation the time-delay measurements from two base stations are transmitted via a radio link or fixed communication lines to a central station. From the time delays distances can be calculated, and using the distances it is possible to generate circles corresponding to loci of possible positions. The intersection of these loci establishes the position of the mobile telephone. There are two possible intersection points. Any ambiguity can be often resolved from a priori information. If this is not possible, a measurement from a third base station will resolve the issue. The third measurement will also allow a higher level of accuracy for the position measurement. Measurements from more than three base stations can also be combined, using standard techniques, to give more accurate measurements.
In the hyperbolic self-positioning system, the mobile telephone will compare the time of arrival of signals from three different base stations. The difference in time of arrival from two of the base stations will define one hyperbola, the time difference between another pair of stations will generate another hyperbola. The intersection of the two hyperbolas will define the location of the mobile telephone. In some cases there will be two intersections raising possible ambiguity. This can be resolved by the use of a fourth base station. This fourth base station will also allow a higher level of accuracy for the position measurement. Measurements from more than four base stations can also be combined to give more accurate measurements.
A combination of other modes is also possible. For example the round trip times could be measured at the mobile, so producing a radial self-positioning system. Alternatively the round trip time measurements could be made at the base stations, but sent to the mobile, which would then make the position calculations; an indirect radial self-positioning system. Alternatively, the base stations could co-operate to measure the time difference of arrival of the signals from a single mobile; a direct remote hyperbolic positioning system.
There are many possible uses for a system that allows accurate location of mobile telephones. These include locating people who are in distress, efficient dispatching of fleets, providing navigational guidance, recovery of stolen telephones, and giving geographically referenced information such as the location of the nearest restaurant. There have already been some attempts at implementation of systems for locating mobile phones. However, as yet there has not been a widespread commercial implementation. The major reason for this is that current solutions have technical deficiencies in the areas of coverage, accuracy, and cost.
In a first aspect the invention provides a mobile radio-frequency communications system, comprising: a network of at least two base stations arranged to transmit/receive signals to at least one mobile radio-frequency transceiver; a reference receiver located at a known distance from the base stations and including measuring means to measure the times of arrival of signals transmitted from the base stations; determination means to use the known distances and measured times to determine the relative time offsets of transmissions from each base station; and location means to use the determined relative time offsets to calculate the position of a mobile transceiver in the network area using hyperbolic positioning techniques.
The invention may be applied to any communications system that uses multiple sites, including cellular telephone systems, personal communications systems, and orbiting mobile telephone systems. It is an advantage of at least some embodiments of the invention to improve the accuracy and coverage, and reduce the cost of locating mobile telephones. Another advantage is the small number of changes required to a cellular system in order to implement some embodiments of the invention.
The position information could be derived remotely by the network or locally by the mobile transceivers.
Such a system could measure the actual propagation time of the signal transmitted from each base station to the reference receiver, compare the actual propagation time with a reference propagation time, and provide the selected mobile transceivers with time-difference measurements. The time difference measurements may include differences introduced by synchronisation errors, and by propagation delays.
Alternatively, the reference radio-frequency receiver may include signal processing means to measure the actual propagation time of the signal transmitted from each base station to the reference receiver and compare the actual propagation time with a reference propagation time. The reference receiver may also communicate the time difference measurements to a control means through its nearest base station, and the control means in turn could forward the time-difference measurements to the base stations for transmission to the selected receivers. In this case the determination means may be situated within the reference receiver, in which case the reference receiver transmits the relative time offsets back to the system.
In a self-positioning system the location means may be situated within the mobile transceiver. Alternatively, in a remote positioning system the location means may be associated with a central site remote from the mobile. For a remote system, the central station could use the information derived from the reference receiver in order to make the position determination.
The location means may receive information from the reference receiver by means of a landline rather than by means of a broadcast transmission. The location means may also receive data concerning at least one of base station oscillator stability, basestation location or the phase difference between two signals having different frequencies that have been transmitted by the same base station, by means of the landline.
Only a parameter of the signal (the epoch difference) needs to be transmitted, not the whole signal which is bandwidth efficient, and also means that it is not essential to compare exactly the same epochs. In many applications, the use of a reference receiver is more efficient, as it does not detract from system capacity and can be set up without any modification to the infrastructure of the cellular system.
A selector means may be operationally associated with the determination means to selectively prevent information concerning the position of a mobile transceiver from being available to that receiver. The base stations or the transceivers, or both, may be modified to ensure that only selected transceivers have available the position information. Non-selected transceivers may still be able to use the system to communicate, but will either not have the position information available to them, or will have position information of lesser accuracy than is available to the selected transceivers. The selected transceivers could include a decoding algorithm that allows only those mobile transceivers to use the time-difference measurements supplied by the system and so calculate their position.
In circumstances where not all the base stations in a system can be seen from one reference receiver, several reference receivers could be used. Provided there is an overlap between the base stations seen by the reference receivers it may be possible to achieve system wide synchronisation.
In a second aspect the mobile transceiver calculates the phase difference between at least two signals having different frequencies that have been transmitted from the same base station in order to calculate its position. The network in this aspect can be a synchronised or a non-synchronised network.
Both the mobile and reference radio-frequency receivers and the basestations may be multi-channel receivers of the type having a timing reference circuit under the control of a processing means. Alternatively, a single channel receiver may be employed which is switchable between the different frequencies used by the base stations in the network.
The system can also include a means of accounting for the distortion to signals transmitted between the base station and the mobile or reference receiver caused by the multi-path reflections. Such reflections are typically caused by buildings and other objects common in urban and suburban areas.
In a preferred arrangement, the process for rejecting signals that have undergone multi-path reflections includes using the estimate of the impulse response from the equaliser of the mobile telephone and using this estimate of the impulse response to reject the multipath reflections and other channel induced errors. This has the advantage of reducing the processing load of the mobile telephone by reusing the equaliser information, so reducing cost, complexity, and power drain.
In a third aspect the invention provides a mobile radio-frequency communication system, comprising: a network of at least three base stations each arranged to transmit/receive signals to/from at least one mobile radio-frequency transceiver; wherein the transceiver(s) calculate the impulse response of the channel and uses this information to reduce multi-path effects.
When a mobile transceiver is moving relative to the base stations, such as when placed in a moving vehicle, it is possible to use the movement of the vehicle to produce a synthetic antenna aperture such an aperture may have a width of many wavelengths. In one embodiment of the present invention, such a synthetic aperture could be used for multi-path rejection by steering the antenna towards the base station. An added source of position information can also be derived from the synthetic aperture by determining the angle of arrival of the signal. The synthetic antenna aperture may provide better signal to noise performance. The MUSIC signal processing algorithm could also be used for processing of the data from this synthetic aperture to improve performance.
In one embodiment of this aspect, any signals arriving at the mobile transceiver that have undergone multi-path reflections are subtracted from the first direct signal to be received using a Fourier transform equalisation technique.
In a fourth aspect of the invention provides a mobile radio-frequency communication system, comprising: a network of at least three base stations each arranged to transmit signals to at least one mobile radio-frequency transceiver; wherein the mobile transceiver synthesises a wide aperture while it is moving to provide additional information for position calculation.
In a fifth aspect the invention provides a mobile radio-frequency communication system, comprising: a network of at least three base stations each arranged to transmit signals to at least one mobile radio-frequency transceiver; wherein the mobile transceiver synthesises a wide aperture while it is moving to provide additional information to reduce multi-path effects.
It would also be possible to use directional antennas, both on the mobile transceiver and each of the base stations, to improve multi-path performance, resolve ambiguity and increase accuracy. Another option could be to use adaptive beam forming to null interferers, such as multi-path signals.
In urban areas, there will commonly be many other radio sources such as commercial radio stations, television stations and analogue mobile telephone services. An advanced receiver could integrate information from all these sources to provide even better position estimates.
In a sixth aspect the invention provides a mobile radio-frequency communication system, comprising: a network of at least two base stations each arranged to transmit/receive signals to at least one mobile radio-frequency transceiver; wherein a directional antenna is used for position ambiguity resolution, position determination or to reduce multi-path effects.
In a seventh aspect the invention provides a mobile radio-frequency communication system, comprising: a network of at least three base stations each arranged to transmit or receiver signals to/from at least one mobile radio-frequency transceiver; wherein Doppler estimates of the velocity of mobile transceivers are used to calculate the velocity of the mobiles.
This velocity information could be used in various ways including improving position information by means of an integrating filter (eg Kalman filter). Because the mobile will often be frequency locked to one of the basestations, the conventional means of velocity estimation will give incorrect estimates.
In an eighth aspect the invention provides a mobile radio-frequency communication system, comprising: a network of at least three base stations each arranged to transmit or receive signals to/from at least one mobile radio-frequency transceiver; wherein information such as map data, dead reckoning, communications broadcasts are used to improve mobile transceiver position estimates.
It would also be possible to integrate a map display with the position data developed by the invention and to use map aided positioning techniques so that the position of the mobile transceiver could be overlaid on a map to allow ready determination of position.
Even with a large number of base stations within a city, there will be some regions where a position measurement will be difficult or not possible. One means of overcoming this difficulty could involve the use of a low cost dead reckoning sensor, such as a compass, integrated into the system.
In the network, each base station can operate with a number of carrier frequencies. In most situations, it would also be possible for any one mobile transceiver to be within transmission range of more than three base stations. As a result, there will be a considerable amount of redundant carrier phase information available, which can be used to improve the accuracy of the system. For example, as Kalman filtering could be applied to the phase and time of arrival information in order to produce optimal filtering of the positioning data. The major advantage of such filtering is that it should be used to refine position estimates even when the mobile transceiver was moving.
The use of differential information derived from a reference receiver could also be used to significantly increase the accuracy of the location of the mobile transceiver.
In a ninth aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a mobile radio-frequency communication system comprising a network of at least three base stations arranged to transmit signals, the method comprising the steps of: locating a reference receiver at known distances from the base stations; measuring the times of arrival of signals transmitted from the base stations; determining the relative time offsets of transmissions from each base station, using the known distances and measured times; and calculating the position of a mobile transceiver in the network area using the determined relative time offsets.
The invention may comprise the further step of selectively preventing information concerning the position of a mobile transceiver from being available to that transceiver. In this way it would be possible to charge the mobile transceiver for the use of this information.
In a tenth aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a communication system comprising a network of at least two base stations arranged to transmit signals at several different frequencies, comprising the step of: calculating the phase difference between the two or more signals having different frequencies that have been transmitted from the same base station in order to calculate its position.
In an eleventh aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a communication system comprising a network of at least two base stations arranged to transmit/receive signals, comprising the step of: calculating the impulse response to the channel and using this information in the mobile transceiver to reduce multi-path effects.
In a twelfth aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a communication system comprising a network of at least three base stations arranged to transmit signals, comprising the step of: synthesising a wide aperture while the mobile transceiver is moving to provide additional information for position calculation.
In a thirteenth aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a communication system comprising a network of at least three base stations arranged to transmit signals, comprising the step of: synthesising a wide aperture while the mobile transceiver is moving to provide additional information to reduce multi-path effects.
In a fourteenth aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a communication system comprising a network of at least three base stations arranged to transmit signals, comprising the step of: using a directional antenna, either at base station or on a mobile transceiver, for position ambiguity resolution, position determination or to reduce multi-path effects.
In a fifteenth aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a communication system comprising a network of at least three base stations arranged to transmit/receive signals, comprising the step of: using Doppler estimates of the velocity of mobile transceivers to calculate the position or location of the mobiles.
In a sixteenth aspect the invention provides a method of finding the position of a mobile radio-frequency transceiver in a communication system comprising a network of at least three base stations arranged to transmit/receive signals, comprising the step of: using information such as map data, dead reckoning, communications broadcasts to improve mobile transceiver position estimates.
The system could also include a means of encrypting the signals transmitted between the base stations and mobile and reference receiver to safeguard privacy.
In one embodiment of this aspect, the network preferably comprises a cellular telephone network with the mobile transceiver comprising a cellular mobile telephone. The mobile telephone preferably operates under the GSM standard or the system based on GSM used in the United States of America.
A possible architecture of the invention, when relying on a remote hyperbolic tracking system, could consist of each base station having multiple receptor modules, one for each frequency being used in that region. The modules that do not correspond to the frequencies being used by a particular base station cell could listen to the mobile transceivers in other cells, measure the time of arrival of those signals, and pass those measurements to the control means for processing. Alternatively, the base station receivers could incorporate a pre-amplifier and one or a series of digital signal processors to simultaneously process the information received.